Carbon dioxide sensitivity was first discovered by L'Heritier and Teissier in 1937 in Drosophila melanogaster (Meigen). Carbon dioxide sensitivity is a genuine case of cytoplasmic inheritance, and the word
"genoide" was coined for the cytoplasmic determinant. T h e viral nature of the so-called "genoide" was demonstrated with the evidence that an infectious agent could be extracted from sensitive flies. When injected into Drosophila from a resistant origin, this agent multiplied, bringing about the development of C02-sensitivity symptoms, but no disease.
Judging from its genetic behavior, its size, the course of its multiplica
tion, and its ability to undergo mutation, the C02-sensitivity agent fits every characteristic of a virus and has been called the sigma virus or virus a.
T h e virus responsible for C02-sensitivity in Drosophila can be trans
mitted by injection from C02-sensitive flies to normal individuals. As stable C02-sensitivity in Drosophila is transmitted only by crosses in which the females carry the character, the inheritance of C02-sensitivity is maternally inherited, and inheritance of this characteristic appears to be controlled by a cytoplasmic localization of the so-called provirus.
When the virus penetrates into an oocyte, the opportunity arises for initiating the so-called stabilized condition, which corresponds to the symbiotic cycle in bacteria.
Such a phenomenon resembles latency of insect-virus infections and the occult insect viruses. A "stabilized fly" is the equivalent of a culture of lysogenic bacteria and, similarly, mature virus can only be produced by an accidental upsetting of the symbiotic hereditary equilibrium, as
in the induction of insect-virus infections. T h e virus remains within the generations of Drosophila cells, without being lost during mitosis and female meiosis and without maturing, and becomes apparent or manifests itself under only rare circumstances (L'Heritier, 1958). T h e integrated complex is very similar to a lysogenic bacterium and insect-virus induc
tion. Sensitive flies seem to live as long as resistant ones and lay eggs normally. Nothing is known of the intimate physiological mechanism underlying sensitivity, since only the very specific action of C 02 has been reported, whereas several physical and chemical stressors or in-citants, are effective for the induction of insect-virus infection and lysogenic bacteria.
T h e case of lysogenic bacteria has been compared with that of latent viral infections in plants and animals (Lwoff, 1953; Jacob, 1954).
Lysogenic bacteria contain a noninfectious factor, the prophage, which is responsible for their ability to produce phage (Lwoff and Gutmann, 1950). Phage development appears to be possible only when the pro-phage-bacterium relationship is disturbed. Phage production by lysogenic bacteria is controlled by three factors: (1) the genetic property of the lysogenic strain, (2) the inducing shock, and (3) the physiological conditions of the culture. These factors are also important in the induc
tion of insect-virus infections. Various physical and chemical agents are able to initiate the development of the prophage in inducible lysogenic bacteria. Both ultraviolet light near 2537Ä (Lwoff and Gutmann, 1950) and soft X rays (Latarjet, 1951) are very efficient. Among the chemicals that have been shown to act as inducers are organic peroxide, epoxide, ethyleneimines (Lwoff and Jacob, 1952) as well as nitrogen mustard
(Jacob, 1952). Hydrogen peroxide also induces phage formation (Lwoff and Jacob, 1952). All the inducing agents are also known to stimulate mutagenic or carcinogenic activities. However, all the mutagenic and carcinogenic substances do not induce phage production in lysogenic bacteria. Some physical and chemical agents effective for the induction of lysogenic bacteria also effectively induce insect-virus infection, as mentioned above.
Another property which is conferred on lysogenic bacteria by the presence of a prophage is their immunity to an infection by the phage they are able to produce, the homologous phage, but such a phenomenon has not been recognized in insects (Aruga et al., 1961). A lysogenic bacterium is always immune against the homologous phage or mutants of this phage, but in B. mori the larvae containing an occult virus which produces hexagonal polyhedra is not immune against both the homol
ogous virus and its mutant, tetragonal polyhedron virus.
V I . RECAPITULATION
Concerning the induction of insect-virus infections it may be con
cluded that there are some differences among insect species, stressors, physiological conditions, and other factors.
(1) There is a marked difference between most insect species and B.
mori, which is a domesticated insect selected for a specific purpose, and which is the most inducible insect among those so far tested. (2) There is a difference in induction among several strains of a species. (3) T h e rate of induction is different at the several developmental stages of insects. (4) Differences between pure lines and hybrids can be observed in some insects in the induction of insect-virus infections. (5) In some insects a parallelism is recognized between the natural occurrence of virus diseases and the rate of induction by some stressors (or by inci-tants), but this is not applicable in all cases investigated. (6) There are differences in the induction of two or more virus diseases among strains of a given insect species. T h e r e is, for example, a strain which is very resistant to nuclear and cytoplasmic polyhedrosis in B. mori, but certain other strains resist the induction of nuclear polyhedrosis, although not that of cytoplasmic polyhedrosis. Such a phenomenon may exist in other insects. (7) There are a few stressors that are able to induce two or more different kinds of insect-virus diseases, but others may induce only one of these virus diseases. (8) In the artificial induction of insect-virus infections by treating insects with some physical and chemical agents, several modifying factors play a part, e.g., rearing temperature, humidity, quality and quantity of food. (9) One of the important factors control
ling the rate of induction is the stage of the development of insect, i.e., molting and pupation. There are marked differences in induction among the several larval and molting stages. Induction may be controlled by the nature of the differentiation of tissue cells and that of physiological function, which, in turn, may be controlled by several hormonal sub
stances secreted by a few organs or tissues at certain critical stages. (10) Physiological functions and their changes in insects (controlled by both the genetic and environmental factors) play an important role in the induction of insect-virus diseases. (11) It is important to discriminate between the induction of virus diseases in which no virus feeding occurs before the treatment by stressors, and the increase of the occurrence of insect-virus diseases in which small amount of viruses are fed before the treatment by some inducing agents, stressor, or incitant. In experiments on the induction of insect-virus diseases, it is necesary to avoid con
tamination by virus particles or polyhedral bodies in the course of the experiments. On the other hand, we must be careful about the influence of changes in physiological conditions, which are controlled by genetic
and environmental factors, and their role in the induction of insect-virus diseases.
T h e state or nature of occult virus in the cells of organs and tissues of insects has not been determined. It is very difficult to determine the nature of occult virus which is presumed necessary for the phenomenon of latency and for the induction of viruses which multiply in the nucleus or cytoplasm. However, a hypothesis has been proposed by Yamafuji, based on the result of chemical inductions of nuclear polyhedrosis in the silkworm; and the relation between the bacterial chromosome and pro
phage in lysogenic bacteria has been discovered. It may be expected in the near future the nature of occult virus and the mechanism of induc
tion of insect-virus infections will be further clarified cytologically and biochemically.
T h e induction of insect-virus infection by chemicals is also significant in respect to the microbial control of insects. I f certain chemicals are found to be not injurious to human beings and domesticated animals, and yet effective in the induction of insect-virus diseases, they can be utilized for the purpose of controlling insects by means of induced infections.
Although it does not seem possible as yet to develop a satisfactorily simple picture of the induction of insect-virus infections, the situation is rather encouraging in the light of recent discoveries and the many interesting experiments that still remain to be carried out.
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